1. Field of the Invention
The invention relates generally to curable silicon-based adhesives. In particular, the invention relates to joining silicon parts used in semiconductor fabrication equipment.
2. Background Art
Batch substrate processing continues to be used in fabricating semiconductor integrated circuits and similar micro structural arrays. In batch processing, many silicon wafers or other types of substrates are placed together on a wafer support fixture in a processing chamber and simultaneously processed. Currently most batch processing includes extended exposure to high temperature, for example, in depositing planar layers of oxide or nitride or annealing previously deposited layers or dopants implanted into existing layers. Although horizontally arranged wafer boats were originally used, vertically arranged wafer towers are now mostly used as the support fixture to support many wafers one above the other.
In the past, the towers have been most often made of quartz or sometimes of silicon carbide for high-temperature applications. However, quartz and silicon carbide have proven unsatisfactory for many advanced processes. An acceptable yield of advanced integrated circuits depends upon a very low level of particles and metallic contaminants in the processing environment. The quartz towers often develop excessive particles after a few cycles and must be reconditioned or discarded. Furthermore, many processes require high-temperature processing at above 1000° C. or even above 1250° C. Quartz sags at these high temperatures although silicon carbide maintains its strength to a much higher temperature. However, the high temperature activates the diffusion of impurities from either the quartz or silicon carbide into the semiconductor silicon. Some of the problems with silicon carbide have been solved by coating a sintered SiC structure with a thin SiC surface layer deposited by chemical vapor deposition (CVD). This approach, despite its expense, has its own problems. Integrated circuits having features sizes of 0.13 μm and below often fail because slip defects develop in the silicon wafer. It is believed that slip develops during initial thermal processing when the silicon wafers are supported on towers of a material having a different thermal expansion than silicon.
Many of these problems have been solved by the use of silicon towers, particularly those made of virgin polysilicon, as described by Boyle et al. in U.S. Pat. No. 6,450,346, incorporated herein by reference in its entirety. A silicon tower 10, illustrated orthographically in FIG. 1, includes three or more silicon legs 12 joined at their ends to two silicon bases 14. Each leg 12 is cut with slots to form inwardly projecting teeth 16 which slope upwards by a few degrees and have horizontal support surfaces 18 formed near their inner tips 20. A plurality of wafers 22, only one of which is illustrated, are supported on the support surfaces 18 in parallel orientation along the axis of the tower 10. For very high-temperature processing, it is preferred that there be four legs 12 and that the support surfaces 18 be arranged in a square pattern at 0.707 of the wafer radius from the center.
Superior results are obtained if the legs 12 are machined from virgin polysilicon, which is silicon formed by chemical vapor deposition from a gaseous precursor, typically silane (SiH4) or a chiorosilane (SiClH3, SiCl2H2, SiCl3, or SiCl4). Virgin polysilicon (virgin poly) is the precursor material used for the Czochralski growth of silicon ingots from which wafers are cut. It has an exceedingly low level of impurities. Although virgin poly would be the preferred material for the bases 14, it is not usually available in such large sizes. Instead, Czochralski or cast silicon may be used for the bases. Their higher impurity level is of lesser importance since the bases 14 do not contact the wafers 22.
Fabricating a silicon tower particularly out of virgin poly requires several separate steps, one of which is joining the machined legs 12 to the bases 14. As schematically illustrated in FIG. 2, mortise holes 24, which are preferably blind but may be through, are machined into each base 14 with shapes in correspondence with and only slightly larger than ends 26 of the legs 12. Boyle et al. favor the use of a spin-on glass (SOG) that has been thinned with an alcohol or the like. The SOG is applied to one or both of the members in the area to the joined. The members are assembled and then annealed at 600° C. or above to vitrify the SOG in the seam between the members.
SOG is widely used in the semiconductor industry for forming thin inter-layer dielectric layers so that it is commercially available at relatively low expense and of fairly high purity. SOG is a generic term for chemicals widely used in semiconductor fabrication to form silicate glass layers on integrated circuits. Commercial suppliers include Allied Signal, Filmtronics of Butler, Pa., and Dow Corning. SOG precursors include one or more chemicals containing both silicon and oxygen as well as hydrogen and possibly other constituents. An example of such as precursor is tetraethylorthosilicate (TEOS) or its modifications or an organo-silane such as siloxane or silsesquioxane. When used in an adhesive, it is preferred that the SOG not contain boron or phosphorous, as is sometimes done for integrated circuits. The silicon and oxygen containing chemical is dissolved in an evaporable liquid carrier, such as an alcohol, methyl isobutyl ketone, or a volatile methyl siloxane blend. The SOG precursor acts as a silica bridging agent in that the precursor chemically reacts, particularly at elevated temperature, to form a silica network having the approximate composition of SiO2.
It is believed that the process produces the structure illustrated very schematically in cross section in FIG. 3. Two silicon members 30, 32 are separated by a gap 34 having a thickness of about 50 μm (2 mils). The thickness of the gap 34 represents an average separation of the leg 12 and the base 14 of FIG. 2 as the end 26 of the leg 12 is at least slidably fit in the mortise hole 24. The gap thickness cannot be easily further reduced because of the machining required to form the complex shapes that guarantee alignment and because some flexing of assembled members is needed to allow precise alignment of the support surfaces and other parts. A coating of SOG is applied to at least one of the mating surfaces before the two members 30, 32 are assembled such that the SOG fills the gap 34 of FIG. 3. Following curing and a vitrification anneal, the SOG forms a silicate glass 36 that is extremely schematically represented in the figure as a three-dimensional network of silicon and oxygen atoms and their bonds. Note that the silicon-oxygen bond lengths are on the order of a nanometer in comparison to the tens of micrometers for the gap. The silicate glass 36 may be referred to as silica having a composition of approximately silicon dioxide (SiO2) and forms as an amorphous solid with most silicon atoms bonding to four oxygen atoms and most oxygen atoms bonding to two silicon atoms. The figure shows oxygen atoms bonding to silicon atoms in the silicon members 30, 32 at the silicon surfaces 38, 40. However, the structure is in reality more complex since the silicon members 30, 32 likely have a thin native oxide layer, that is, of SiO2 at their surfaces 38, 40. The vitrification anneal rearranges some of the oxide bonds to bond instead to oxygen or silicon atoms in the silica glass 36.
Silicon towers produced by this method have delivered superior performance in several applications. Nonetheless, the bonded structure and in particular the bonding material may still be excessively contaminated, especially by heavy metal. The very high temperatures experienced in the use or cleaning of the silicon towers, sometimes above 1300° C., may worsen the contamination. One possible source of the heavy metals is the relatively large amount of SOG used to fill the joint between the members to be joined. Siloxane SOG is typically cured at around 400° C. when used in semiconductor fabrication, and the resultant glass is not usually exposed to high-temperature chlorine. However, it is possible that the very high temperature used in curing a SOG adhesive draws out the few but possibly still significant number of heavy metal impurities in the SOG.
Furthermore, the joints joined by SOG adhesive have not proved as strong as desired. Support towers are subject to substantial thermal stresses during cycling to and from high temperatures, and may be accidentally mechanically shocked over extended usage. It is greatly desired that the joints not determine the lifetime of the support tower.